A number of quinolone antibiotics are known, including commercially available ones, but they involve certain problems such as the fact that these compounds must be used with utmost caution because many of them show side-effects in the central nervous system. Recently, much attention has been paid to the antibacterial activity of quinoline derivatives that have a fluorine substituent at both 6- and 8-position, or a fluorine substituent at 6-position and a lower alkoxy substituent at 8-position (U.S. Pat. No. 4,556,658, European Patent No. 106,489, European Patent Laid Open No. 230,295, European Patent Laid Open No. 241,206). However, they are not always satisfactory antibiotics, since many of them have phototoxicity along with the side-effects mentioned above.